1. Field of the Invention
The present invention relates to oil field downhole tools. Particularly, the invention relates to flow control valves used in tubulars in a wellbore.
2. Background of the Related Art
In the operation of oil and gas wells, it is often necessary to enter the wellbore to perform some downhole task. Tool retrieval, formation stimulation and wellbore clean out are all examples of tasks carried out in a live well to improve production or cure some problem in the wellbore. Typically, a tubular of some type is inserted into a wellbore lined with casing or is run in production tubing to perform these tasks. Because so many wells are located in remote locations, coil tubing is popular for these operations because of its low cost and ease of use compared to rigid tubulars.
Selectively pumping a pressurized liquid or gas into a live well presents some challenges regardless of the use of rigid or coil tubing. For example, most operations require the fluid to be pumped at a predetermined depth in order to effect the right portion of a formation or to clean the effected area of the wellbore. In order to maintain the liquid in the tubular until a predetermined time, a valve proximate the downhole end of the tubular string is necessary to prevent the fluid from escaping until the operation begins. Additionally, to prevent loss of pressure in the tubular, the valve must open and close rapidly. The rapidity of operation is especially critical when coil tubing is used, because the maintenance of pressure within the coil tubing is necessary to prevent the tubing from collapsing due to adjacent pressure in the wellbore.
FIG. 1 is an exemplary well 10 which could be the subject of a downhole cleaning, removal or formation perforation operation. Typically, the wellbore hole is cased with a casing 12 that is perforated to allow pressurized fluid to flow from the formation 18 into the wellbore 15. To seal the mouth of the well, a wellhead 20 is mounted at the upper end of the wellbore. The wellbore in FIG. 1 is shown with a string of coil tubing 14 inserted therein. As herein described, the tubing is typically filled with a liquid or gas, such as water, foam, nitrogen or even diesel fuel for performing various operations in the well, such as cleaning or stimulating the well.
The weight of the fluid in the tubular member 14 creates a hydrostatic pressure at any given depth in the tubular member. The hydrostatic pressure in the tubing at the top surface is approximately zero pounds per square inch (PSI) and increases with depth. For example, the hydrostatic pressure caused by the weight of the fluid in the tubing in a 10,000 feet deep well can be about 5,000 PSI. In many instances, the hydrostatic pressure at a lower zone 22 of the tubing is greater than the wellbore pressure at a similar depth in the wellbore zone 24. Thus, a flow control valve 16 is used to control or stop the flow of the fluid from the tubular member 14 into the wellbore 15.
Even though the hydrostatic pressure in the tubing can be greater than the wellbore pressure near the bottom of the well, the opposite effect may occur at the top of the well. If the wellbore pressure is high, for example, in a gas well, the wellbore pressure at the top of the well can be several thousand PSI above the relatively low hydrostatic pressure in the tubing at the top of the well. It is generally known to well operators that a wellbore pressure greater than about 1,500 PSI can crush some tubing customary used in well operations, such as coil tubing. Thus, operators will pressurize the tubing 14 with additional pressure by pumping into the coil tubing to overcome the greater wellbore pressure at the top of the wellbore. In some high differential pressure applications, fluid must be pumped continuously through the tubular to maintain a pressure at the top of the tubular and waste the fluid into the wellbore because of the inability of a valve to control the high differential pressures.
In other applications, such as in lower differential pressure applications, a flow control valve can be mounted to the end of the tubular to attempt to adjust for the differences between the downhole hydrostatic pressures and associated wellbore pressures. The valve allows the wellbore pressure to counteract the hydrostatic pressure in conjunction with an upwardly directed spring force. FIG. 2 is a schematic of one exemplary differential flow control valve. The valve 26 is disposed at the lower end of a tubing (not shown) and has an upper passageway 28 through which tubing fluid can flow. The lower passageway 29 of the valve 26 allows wellbore fluid at a wellbore pressure to enter the valve 26. A poppet 30 is disposed within the valve 26 and engages a seat 32. Belleville washers 34, acting as a disk shaped spring, are disposed below the poppet 30 to provide a sufficient upward bias to override the hydrostatic pressure in the passageway 28. When the sealing member is sealingly engaged with the seat 32, the two passageways are fluidly disconnected from each other. When the pressure is increased sufficiently to override the upward bias, the sealing member 30 separates from the seat 32 and the two passageways are in fluid communication. The valve 26 operates on differential pressures in that the wellbore pressure provides an upward force on the poppet in addition to the Belleville washers 34.
However, it has been discovered that while the Belleville washers can open quickly, the washers close slowly, i.e., operate with different opening and closing speeds, known as a hysteresis effect. Thus, the valve 26 can be opened to flow pumped fluid from the tubing 14 into the wellbore 15 (shown in FIG. 1), but is insufficient to quickly close the valve to retain pressure in the tubing once a pump has stopped pumping fluid into the tubing to allow the valve to close. Thus, the differential pressure at the upper portion of the tubing is not maintained and the tubing can be deformed or crushed when a high differential pressure exists between the inside of the tubing and the surrounding wellbore. Other manufacturers, such as Cardium Tool Services, use a coil spring in a hydrostatic valve, but enclose the coil spring in a sealed chamber that is not open to varying pressures and thus not a differential flow control valve. Such valves can collapse and seize when high differential pressures are encountered.
It would be desirable to use a coil spring in a differential flow control valve, which has less hysteresis effects and generally equal opening and closing speeds, but the required forces generated from a typical coil spring in the relatively small diameters of the valve are insufficient to simply replace the Belleville washers. Thus, the use of a coil spring is not practical in a typical differential flow control valve.
Thus, there exists a need for a differential flow control valve which is more responsive to hydrostatic pressures, especially in applications having a high hydrostatic pressure compared to a surrounding wellbore pressure.
A downhole differential flow control valve is provided that utilizes a differential pressure area having one pressure area on which the wellbore pressure acts and a second area different from the first area on which pressure in the tubing acts. The differential area reduces the load in which the spring is required to exert a closing force in the valve. Thus, a coil spring can be used to improve the closing speeds of the valve.
In one aspect, a valve is provided for use in a wellbore, the valve comprising a body, a piston disposed in the body for engaging a valve seat disposed in the body, a biasing member producing a spring force to urge the sealing end of the piston into engagement with the valve seat, whereby the valve opens when the second force exceeds a combination of the spring force and the effective force. In another aspect, a differential pressure control valve is provided for oil field applications, comprising a valve housing having a housing passageway, a valve seat coupled to the housing and having a seat passageway disposed therethrough, a sealing member at least partially disposed within the valve housing and selectively engagable with the valve seat, a bias cavity in fluid communication with the seat passageway; and a bias member coupled to the sealing member that biases the sealing member toward the valve seat. In another aspect, a method of actuating a differential flow control valve is provided, comprising allowing a sealing member to engage a seat on a first piston surface, allowing a first fluidic pressure to apply a first force on at least a first portion of the first piston surface while allowing the first fluidic pressure to apply a greater force on a second piston surface distal from the first piston surface, biasing the sealing member toward the seat with a bias member having a cavity in fluidic communication with the first fluidic pressure, and applying a second fluidic pressure to at least a second portion of the first piston surface to open the valve, wherein a cross sectional area of the second portion is greater than a cross sectional area of the first portion.